In steel cord applications for rubber reinforcement in general, there is a tendency to the use of higher tensile strength cords, making use of higher tensile strength steel filaments.
For off-the-road tires and conveyor belts, a steel cord comprising different layers of steel elements are used. One or more layers of steel elements surround a steel cord, which is called “metal core strand”. These steel elements may be steel cords or strands, so providing e.g. a typical off-the-road steel cord construction 7×(3+9). The elements may also be steel filaments, so providing concentric layered cords, e.g. 3+9+15.
The use of high tensile strength steel filaments for these types of steel cord have, however, a disadvantage. A higher loss of tensile strength is noticed due to the twisting step or steps during transformation of the steel filaments into stands, multi-strands or concentric layered cords, when such high tensile strength steel filaments are used. When strands, multi-layered strands or concentric layered cords are subjected to an axial load, the different filaments of these strands or cords exercise radial forces to each other. They, so-to-say clamp each other. It was found that the higher the tensile strength of the filament, the larger the loss of tensile strength under simultaneous radial and axial load.
This explains the fact that the higher the tensile strength of the filament, the larger the loss of tensile strength due to the twisting steps used to transform the filaments into a strand or concentric layered cord. Further, the more complex the construction of the multi-strands or concentric layered cords, the larger the loss of tensile strength. Especially when the twisting directions of the different layers are different. E.g. a metal core strand twisted in Z-direction, a first layer of steel filaments twisted around this metal core strand in S-direction and a second layer twisted around both underlying layer and metal core strand again in Z-direction.
As a result, the use of high tensile strength steel filaments usually result in moderate or normal levels of tensile strength and breaking load of the provided steel cord or strand, whereas the use of high tensile strength steel elements would suggest higher tensile strengths and breaking loads for the steel cord or strand.
An attempt to reduce this loss of tensile strength has been made by providing steel cords comprising high tensile strength steel filaments, which allow full rubber penetration. However, the results were not fully adequate.